Centrifugal pumps are commonly used to transport or deliver liquids under pressure for applications such as for the transport of as industrial process liquids and the like. The magnetically-driven centrifugal pump is a variant of the centrifugal pump that, instead of having a pump impeller directly attached to a drive shaft, has a separate drive motor and pump rotor. A magnetic coupling between the drive motor and pump rotor is accomplished by arranging a driving magnet concentric to and outside of an annular impeller magnet provided in an impeller. The driving and impeller magnets are magnetically coupled together to transmit rotating torque therebetween, thereby causing the impeller to rotate and effect pressurized transport therefrom.
The advantage of using such a magnetically-coupled or magnetically-drive centrifugal pump design is that it avoids the presence of a drive shaft extending through the pump housing to the pump impeller, thereby avoiding a potential liquid leak path from the pump or impeller cavity. Avoiding such potential leak path is important because it not only minimizes the possibility of a health danger or an environmental hazard in the event of liquid leakage from the pump, but because it also eliminates the potential that the process liquid will be contaminated by contact with the metallic drive shaft element in the event that such liquid leakage occurs.
The desire to avoid liquid leakage and potential process liquid contamination in a pump is especially important in such applications where the liquid being transferred is a high-purity liquid, i.e., process chemicals or water that are used in the semi-conductor manufacturing industry, that must maintain a high degree of chemical purity to avoid contamination that may occur on the microscopic level. Other applications where the need to avoid process liquid contamination include those where the liquid being transported is a chemical medication or liquid meant for human consumption, such as beverages or the like.
Conventional magnetically-driven centrifugal pumps include a pump impeller that is disposed within an impeller cavity of the pump body. The spatial relationship of the pump impeller to the impeller cavity wall is fixed, thereby fixing the tolerance between the impeller and the wall, and fixing the impeller cavity volume. The design of having a fixed tolerance between the impeller and cavity causes the maximum output pressure of the pump to be fixed. In order to obtain a desired maximum pump output pressure it is necessary that the members of the pump be manufactured and assembled within a tight tolerance, and more specifically that the impeller and impeller cavity wall have a zero tolerance fit. Variations in the axial placement of the pump impeller vis-a-vis the cavity wall that are caused either during the manufacturing or assembly stage impact the impeller to cavity wall tolerance, thereby impacting the maximum pump output pressure. Accordingly, it is not uncommon for pumps designed in such manner fashion to have different maximum output pressures.
Additionally, there are applications where small adjustments in the pump output pressure, that are not otherwise achievable by regulating the rotational speed of the drive motor, would be desired. Conventional magnetically-driven centrifugal pumps having a fixed impeller to impeller wall tolerance are not capable of offering a user the option of making such small or finely-tuned pressure adjustments independent of the drive motor.
It is, therefore, desirable that a magnetically-driven centrifugal pump be constructed having means for adjusting the pump output pressure, to achieve maximum output pressure, that operate independent of adjustments made to the drive motor. When transporting high-purity liquids, it is desired that such magnetically-driven centrifugal pump be made in a manner that both: (1) eliminates the possibility that contaminants may be introduced into the process caused by contact of the process liquid with elements of the pump during passage therethrough; and (2) minimizes the number of potential liquid leak paths therethrough, thereby reducing or eliminating the possibility of process liquid escaping from the impeller cavity into other portions of the pump or into the environment. It is desired that such magnetically-driven centrifugal pump have wetted members made from material having a high degree of chemical resistance and/or thermal resistance to resist degradation through contact with corrosive, or caustic chemicals and the like. It is desirable that such magnetically-driven centrifugal pump be capable of operating at high pressures without danger of pump failure or chemical leakage. It is also desirable that such magnetically-driven centrifugal pump be constructed using conventional manufacturing principles from available materials to reduce the cost of manufacturing such pump.